My rod has shrunk

[Manny]

Just a warning if you work from drawings on paper.

I usually draw up rods on hardboard or ply but not having any of suitable size I used cartridge paper 130gsm for various windows I'm making. Started to mark up stiles and then injured my back, so left things for a week.
When I came back to the job the measurements on the rod were different from the stiles already marked up, by about 2mm over 1.2mtr. the paper had shrunk during the week. I used to work in the print so I'm aware that paper moves across the grain but should be stable along the grain.

John

 

[tim]

I had something similar happen when doing some work for a designer. He printed out 1:1 plans for me from autocad but the drawings were a couple of mm out over the 2m length. After much head scratching we reckoned it must have been the plotter motor/ cog wheel slipping. Could well have been shrinkage though.

Cheers

Tim

 

[RogerS]

After much head scratching we reckoned it must have been the plotter motor/ cog wheel slipping.

Tim

Don't think so...relationship between screen drawings and the printout device are non-linear. Try creating a line with your drawing package of known length, print out on your printer then measure!

I have to adjust my scaling on the laser printer to 97% to get an accurate size but even then it's not necessarily correct along the other axis!

 

[Steve Maskery]

Don't think so...

No, I don't either. It was probably that the plotter had not been calibrated to sufficiently high accuracy. When it was calibrated it was probably to a size much smaller than 2m.

My 2p.

Cheers
Steve, whose rod is perfectly sized, thank you.

 

[johnd]

So has mine but I put that down to the medication.

In the case of the drawing I'm pretty sure it was moisture evaporation which would be evident over a long run of paper, wall paper does the same when it dries out.

In print outs its caused by calibration error, try re calibrating the print heads, though I'm certain that that much error would cause any image to appear unsharp (out of focus) but the untrained eye would not notice it on a small print.

 

[tim]

Re the printing prob - it happened 3 years ago....

 

[jasonB]

Thats why you see the words "do not scale from drawing" printed in the heading block, just imagine the error if that 2mm had been at a scale of 1:100.

Same reason that polyester drafting film was used as it didn't move like tracing paper and why good scale rules are made to BS 1347 which has a set amount of movement over a specified temp range.

Jason

 

[Losos]

Many years ago I experienced the frustrations of using a Xerox photocopy which turned out to be inaccurate & subsequently learned that you can't do that When printers came on the scene I made a few test print outs but couldn't get it right so I've given up trying now.

Like the man said, they don't say "Do not scale from print" just because someones got a few minutes to spare

 

[Rgurney]

You could print your rods on Mylar. Mylar doesn't change dimensionally.

 

[Scrit]

This is all making a very good case for NOT using the high tech approach and just stepping out the measurements on a proper rod with an etched steel rule and a pair of dividers. Low tech, but effective. Two steps forward?.......

Scrit

 

[Rgurney]

Scrit,

You're very right. I would like to hear how you layout method for a piece of furniture. Do you use a rod for "one-offs" or just measure them out?

Mylar can be very good for chairs or complicated layouts whether you use CADD or hand draughting.

 

[Scrit]

Do you use a rod for "one-offs" or just measure them out?

Kitchens are a "one off" but they still go on a rod. But for some things I just set up the panel saw stops, the mortiser or whatever and cut. For most pieces of furniture you will have two mirror image sides, so a rod serves you well there and if you aren't going to use the rod again it can be sanded or painted for reuse or used to stoke the fire.

Chairs are a bit different. I've made tub chair frames where I started with full size component templates on thin plywood with the joints marked out on the templates (still two mirror image arms, but a single back piece) - much more available than mylar in my shop! Also the plywood can be quickly and accurately traced round with a pencil for roughting out on the bandsaw. If a more permanent template is required then I've used either galvanised steel or solid HPL in the past.

Scrit

 

[Rgurney]

Scrit,

I've been toying with an idea of using a trammel head for my layout instead of a rod. For "one-offs" you would have two or three key dimensions. For instance, marking the legs would mean marking off the length of the leg plus a key dimension for the bottom of a drawer. All other dimensions would come from the actual parts themselves. Mortise heights aren't crucial and can be marked with a pencil.

As far as the width of the piece is concerned, there would typically be only a couple of shoulder lines.

I think, for one-off fairly simple pieces, using a trammel with a few heads on it would be a quicker layout method and be quite accurate.

Any thoughts?

Regards,

Robert Gurney
Calgary, Alberta, Canada

 

[Scrit]

Part of the reason for using a rod is to retain the settings between sessions or between chairs - which could be made hours, days, weeks oe even minths apart. Trammels or dividers are all very will for transferring dimensions during a session, but I wouldn't want to depend on them staying the same between sessions (trade shop, therefore potentially more than one user, things get dropped, etc). I've not done too many chairs, but when I have done them I've generally made-up accurate full-size templates (normally with the thickness noted on the template in indelible marker) because chairs tend to defy conventional rods in so many areas. It's just the way I was shown to do them and it seems to work quite well for me.

Scrit